Comments by Jan

  • Understanding battery capacity: Ah is not A

    Understanding battery capacity: Ah is not A

    - 22 May 2017

    Hi.

    It looks like you either did not read the blog post or did not get the point of it. 20A is about how fast you can get the energy out, not the capacity of the cell. 3.7V and 2.2 Ah give you 8.14 Wh per cell, and that multiplied by 1234 gives you just over the 10 kWh.

    - Jan

  • Understanding battery capacity: Ah is not A

    Understanding battery capacity: Ah is not A

    - 16 May 2017

    Hi.

    A 12V adapter connected directly to the battery is unlikely to be an appropriate charger. A 12V battery showing you 9V is definitely not good and charged. I am not sure how you are measuring the 0.2amps, that will be a function of your load, and a good battery should definitely be able to supply much more than that. The voltage will gradually go down as your battery discharges. You can probably find a datasheet for the battery you are using and get more specific information. You will need to regulate to 5V anyway, so your main concern should be not discharging your battery too much. And you should get a switching regulator, which should need less than 500mA to output 1A when going from 12V to 5V.

    - Jan

  • Balboa is here!

    Balboa is here!

    - 4 April 2017

    Thanks, and thanks for sharing your robot! The most difficulty came from wanting the Balboa to be able to pop up on its own. I will write up a blog post about it once I find more old prototypes to take pictures of (this project has been going on for maybe eight years, during which we moved several times).

    - Jan

  • Servo control interface in detail

    Servo control interface in detail

    - 20 January 2017

    Kenneth,

    I don't think I implied what you seem to think I did. Did you see the post before this one?

    https://www.pololu.com/blog/16/electrical-characteristics-of-servos-and-introduction-to-the-servo-control-interface

    I think it is not helpful to classify the servo control signals as those in 72 MHz systems vs those in 2.4 GHz systems since that is just the frequency that the transmitter uses to communicate with the receiver and says nothing about what kind of signal the receiver is outputting to the servos. There can of course be differences from brand to brand and unit to unit. You should definitely just look at your particular signals with a scope.

    - Jan

  • Understanding battery capacity: Ah is not A

    Understanding battery capacity: Ah is not A

    - 3 January 2017

    Hello.

    The math stays the same. An 18 Ah battery should provide 18 A for about an hour, so it should be safe if you only need it to last half an hour.

    - Jan

  • Understanding battery capacity: Ah is not A

    Understanding battery capacity: Ah is not A

    - 18 January 2016

    Your math about how long the batteries would last is fine, but the point is you should avoid putting your battery packs in parallel. You should try to use bigger cells if you really need the 5Ah in one pack, or you could put three fans on each pack and have two parallel systems.

    If you really want a single pack of eight AA cells, you could put them all in series to make a 9.6V pack, then put pairs of fans in series and power them from the higher voltage. I don't know about the details of the fans, so this could be tricky in the same way that putting batteries in parallel is tricky: it's difficult to ensure the power will get shared evenly. For instance, if the fan is just a motor and you stall one of the series pair, the voltage across it would go to 0V and the other fan would get the full battery voltage of around 9.6V, which might damage it.

    - Jan

  • On losing my baby

    On losing my baby

    - 31 December 2015

    It got a lot better, but now it's getting worse as we approach his birthday. There are so many triggers for those sad memories. We haven't made much progress on the core conflicting goals of not being sad and not forgetting him.

    Thanks for your thoughts.

    - Jan

  • Understanding battery capacity: Ah is not A

    Understanding battery capacity: Ah is not A

    - 15 December 2015

    Amps multiplied by volts gives you watts. For the same current (amps), higher voltage gets you more power (watts). Factoring in the time (hours) just takes you from an instantaneous rate (watts) to total capacity or energy (watt-hours).

    An easy way to see this intuitively is if you think of two 6V, 51Ah batteries. You would need two of them in series to get to 12V and 51Ah. A single battery has 6V times 51Ah = 306Wh of energy in it. If you have two of the batteries, you have twice as much energy stored, which is also what you get when you multiply 12V by 51Ah to get 612Wh.

    - Jan

  • Continuous-rotation servos and multi-turn servos

    Continuous-rotation servos and multi-turn servos

    - 5 December 2015

    Hello, juerg.

    I addressed the reasons for some of these hobby servo limitations in the first post of this series, Introduction to servos (see the "Ramifications of the servo’s intended use" section):

    https://www.pololu.com/blog/12/introduction-to-servos

    The short answer is that the applications these hobby servos are designed for do not need the features you are looking for, and adding them would make them much more expensive. The main feature most of us want from a servo is to hold absolute position, and incremental encoders are not enough if you don't have the option to go to some home position as part of a power-up sequence. High-resolution, absolute position encoders are expensive, and even then, how would the servo know if it is at, say, 10 degrees or 370 degrees when it turns on?

    There are some fancier servos for robots that might work for your application. For instance, the Dynamixel servos specify a 300 degree position feedback range and also have a continuous rotation mode. I have not played with one and don't know how easy it is to transition between modes and how it behaves when you do, but maybe that could work for you if you never need to do position control within that 60-degree blind region. I suspect there are some robot hobby servos that have continuous positioning over the full 360 degrees, but I do not specifically know of one.

    You might also get a decent servo that will get you at least 150 degrees or so and then use an external 1:3 gearbox to get triple the range (and 1/3 the torque and resolution). Servo City has some external gearboxes for servos, though they might mostly be intended for increasing torque, not reducing it and increasing range.

    - Jan

  • Understanding battery capacity: Ah is not A

    Understanding battery capacity: Ah is not A

    - 27 August 2015

    Some of the comments and questions have been tending toward subjects like chargers that are beyond the scope of this discussion, which is supposed to be about understanding units for battery capacity such as Ah and Wh. Unfortunately, I do not have time to be answering all these questions, so I will generally not respond to questions about particular battery-related products.

    - Jan

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